they are both very similar, the geforce certainly isn't powerful enough to utilise all of the memory that it has. If you are serious about your gaming I would look powerful graphics card than either of them. Neither will be able to play any modern games at anywhere near their maximum settings. Perhaps look as fitting a second hand geforce 480, 560 or 570 if your budget is limited.
Amazon Fire HD 8 review: A brilliant combination of function and value – with one massive caveat